Add comment

Author: weswigham

src/services/utilities.ts

@@ -1174,6 +1174,10 @@ namespace ts {
 
             const children = n.getChildren(sourceFile);
             const i = binarySearchKey(children, position, (_, i) => i, (middle, _) => {
+                // This last callback is more of a selector than a comparator -
+                // `EqualTo` causes the `middle` result to be returned
+                // `GreaterThan` causes recursion on the left of the middle
+                // `LessThan` causes recursion on the right of the middle
                 if (position < children[middle].end) {
                     // first element whose end position is greater than the input position
                     if (!children[middle - 1] || position >= children[middle - 1].end) {